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A P I PUBL+534 95 m 0732290 0542770 730 m
Heat Recovery Steam Generators
API PUBLICATION 534 FIRST EDITION, JANUARY 1995
American Petroleum Institute 1220 L Street, Northwest
Washington, D.C. 20005 4
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APL PUBL*534 95 m 0732290 0542773 677 m
Heat Recovery Steam Generators
Manufacturing, Distribution and Marketing Department
API PUBLICATION 534 FIRST EDITION, JANUARY 1995
American Petroleum Institute
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A P I PUBLX534 75 m 0732270 0542772 503 M
I . API PUBLICATIONS NECESSARILY ADDRESS PROBLEMS OF A GENERAL
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Copyright O 1995 American Petroleum Institute
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API P U B L X 5 3 4 95 0732290 0542773 4 4 T
FOREWORD
API publications may be used by anyone desiring to do so. Every
effort has been made by the Institute to assure the accuracy and
reliability of the data contained in them; however, the Institute
makes no representation, warranty, or guarantee in connection with
this pub- lication and hereby expressly disclaims any liability or
responsibility for loss or damage re- sulting from its use or for
the violation of any federal, state, or municipal regulation with
which this publication may conflict.
Suggested revisions are invited and should be submitted to the
director of the Manufac- turing, Distribution and Marketing
Department, American Petroleum Institute, 1220 L Street, N.W.,
Washington, D.C. 20005.
iii
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A P I PUBL*534 75 0732290 0542774 386
CONTENTS Page
SECTION 1 4 E N E R A L 1.1 Scope
................................................................
1.2 Referenced Publications
................................................ 1.3 Definition of
Terms ....................................................
SECTION 2-HRETUBE HEAT RECOVERY STEAM GENERATORS
2.1 General
.............................................................. 2.2
Application
........................................................... 2.3
System Consideration
.................................................. 2.4 Advantages
of Firetube Over Watertube HRSGs .......................... 2.5
Disadvantages of Firetube Relative to Watertube HRSGs . . . . . . .
. . . . . . . . . . . . 2.6 Mechanical Description
................................................ 2.7 Operations
Description .................................................
SECTION 3"VERTICAL SHELL/TUBE WATERTUBE HRSGS 3.1 General
.............................................................. 3.2
Application
........................................................... 3.3
System Consideration
.................................................. 3.4 Advantages
of Vertical Shellnube Watertube Over Firetube HRSGs ......... 3.5
Mechanical Description
................................................ 3.6 Operations
Description .................................................
SECTION &WATERTUBE COILS INSIDE PRESSURE
4.1 General
.............................................................. 4.2
Application
........................................................... 4.3
System Consideration
.................................................. 4.4 Mechanical
Description ................................................ 4.5
Operations Description
.................................................
VESSELS
SECTION 5-WATERTUBE LOW PRESSURE CASING HRSG 5.1 General
.............................................................. 5.2
Application
........................................................... 5.3
System Consideration
.................................................. 5.4 Advantages
........................................................... 5.5
Disadvantages
........................................................ 5.6
Mechanical Description
................................................ 5.7 Operations
Description .................................................
SECTION &HEAT PIPE HRSGs 6.1 General
.............................................................. 6.2
Application
........................................................... 6.3
System Consideration
.................................................. 6.4 Advantages
........................................................... 6.5
Disadvantages
........................................................ 6.6
Mechanical Description
................................................
APPENDIX A-STEAM DRUMS
........................................... APPENDIX B-HEAT FLUX
AND CIRCULATION RATIO ................... APPENDIX C-SOOTBLOWERS
...........................................
V
~
1 1 1
2 2 2 4 5 5
12
13 13 14 15 16 16
16 16 16 17 17
18 18 18 19 19 19 25
25 26 26 28 28 28
31 37 41
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A P I PUBL8534 95 m 0732290 0542775 232 m
Tables 1-Extended Surface Metallurgy
.......................................... 23 2-Fin Design
...........................................................
3-Maximum Gas Velocities
.............................................. 25
Associated Steam Purity at Steady State Full Load Operation
............ 32 A-2-Suggested Water Quality Limits
...................................... 33 B-1-HRSG Firetube and
Watertube Local Heat Flux ........................ 37
23
A-1-Watertube Boilers Recommended Boiler Water Limits and
Figures 1-Horizontal Firetube with External Drum HRSG
.......................... 2 2-Vertical Firetube with External Drum
HRSG ............................. 3 3-Kettle HRSG
......................................................... 4
&Insulated Metal Ferrule
................................................ 6 5-Insulated
Ceramic Ferrule ............................................. 6
6"Conventional Strength Weld
........................................... 7 7-Full Depth Strength
Weld .............................................. 8
&Channel-Tubesheet-Shell Interconnection
................................ 9 9-Dual Compartment Firetube HRSG
..................................... 10 10-Two Tube Pass Firetube
HRSG ........................................ 10 1 I-Internal
Bypass System with Valve and Damper ......................... 11
12-Partially Tubed Firetube HRSG
....................................... 12 13-Vertical Watertube
Floating Head HRSG ............................... 14 1
&Bayonet Exchanger HRSG
............................................ 15 15-Pipe Coil HRSG
Inside a Pressure Vessel ............................... 17
16-Basic Tubular Arrangement
........................................... 19 17-Interlaced
Tubular Arrangement ....................................... 20
18-Natural Circulation HRSG
............................................ 21 19-Typical Gas
Turbine Exhaust Gas HRSG ............................... 21
20-Typical Convection Section HRSG
.................................... 22 21-Recommended Minimum
Metal Temperature ........................... 23 22-Heat Pipe HRSG
.................................................... 26
23"Operating Temperature Range for Typical Heat Pipe Working Fluids
....... 27 24-Heat Pipe Installation
................................................ 28 A-1-Typical
Steam Drum ................................................ 31
B-1-Typical Watertube HRSG
............................................ 38 B-2-Typical
Circulation Rate ............................................. 39
B-3-Typical Forced Circulation System
................................... 39
(Bare or Finned)
.................................................... 41 C-2-Qpical
Fixed-Position Rotary Mounting Arrangement .................. 42
C-3-Steam Flow Rate for Rotary Sootblowers
.............................. 43 C-&Air Flow Rate for Rotary
Sootblowers ................................ 44 C-5-Typical
Retractable Mounting Arrangement ........................... 44
C- 1 Sootblower Cleaning Lanes for Square and Triangular Pitch
Tubes
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A P I PUBL*534 75 m 0732270 O542776 L57 m
Heat Recovery Steam Generators
SECTION 1-GENERAL
1.1 Scope This publication provides guidelines for the selection
or
evaluation of heat recovery steam generator (HRSG) systems.
Details of related equipment designs are considered only where they
interact with the HRSG system design. This publication does not
provide rules for design, but indicates areas that need attention
and offers information and descrip- tion of HRSG types available to
the designer or user to aid in the selection of the appropriate
HRSG system.
The HRSG systems discussed are those currently in industry use.
A general description of each HRSG system begins Sections 2 through
5. Selection of an HRSG system for description does not imply that
other systems are not available nor recommended. Many individual
features de- scribed in these guidelines will be applicable to any
type of HRSG system.
Appendices A , B, and C refer to Sections 1 through 6.
1.2 Referenced Publications 1.2.1 The editions of the following
standards, codes, and specifications that are in effect at the time
of publication of this publication shall, to the extent specified
herein, form a part of this publication.
ABMA
1.2.2 In addition, this publication draws upon the work
presented in the following publications:
Steam: Its Generation and Use, Babcock & Wilcox Company, New
Orleans, Louisiana.
Combustion Engineerin- Reference Book on Fuel Burning and Steam
Generation, Combustion Engineering Co., Inc., Stamford,
Connecticut.
A. Bar-Cohen, Z. Ruder, and P. Griffith, Circumferential Wall
Temperature Variations in Horizontal Boiler Tubes, International
Journal of Multiphase Flow, Vol. 9, No. 1, Massachusetts Institute
of Technology, 1983.
B.Y. Taitel and A.E. Dukler, A Model for Predicting Flow Regimen
Transitions in Horizontal and Near Horizontal Gas-Liquid Flow,
AICHE Journal, Vol. 22, No. 1, University of Houston, January
1976.
1.3 Definition of Terms 1.3.1 Heat recovery steam generator
(HRSG)-A system in which steam is generated and may be superheated
or wa- ter heated by the transfer of heat from gaseous products of
combustion or other hot process fluids.
Recommended Boiler Water Limits and Associated Steam .3.2
Firetube HRSG-A shell and tube heat exchanger in
which steam is generated on the shell side by heat transferred
Purity
ANSIZ/ASME from hot fluid flowing through the tubes. PTC 4.4 Gas
Turbine Heat Recovery Steam Generators
Pe$ormance Test Code 1.3.3 Heat pipe HRSG-A compact heat
exchanger con- sisting of a pressure vessel and a bundle of heat
pipes. The heat pipes extract heat from a hot fluid and transport
it into ASME3
Boiler andPressure Vessel Code, Section I , Power Boilers a
where is generated. and Section W, Division 1, Pressure
Vessels.
ASTM4 Standards for Tubes, Sampling, and Testing
TEMA5 Standards of the Tubular Exchanger Manufacturers
Association (seventh edition)
American Boiler Manufacturers Association, 950 North Glebe Road,
Arlington, Virginia 22203. *American National Standards Institute,
11 West 42nd St., 13th Floor, New York, New York 10036-8002.
3American Society of Mechanical Engineers, 345 East 47th Street,
New York, New York 10017-2392. 4American Society for Testing and
Materials, 1916 Race Street, Phila- delphia, Pennsylvania
19103-1187. 5Tubular Exchanger Manufacturers Association, 25 North
Broadway, Tarry- town, New York 10591-3201.
1.3.4 Vertical shell and tube watertube HRSG-A shell and tube
heat exchanger in which steam is generated in the tubes by heat
transferred from a hot fluid on the shell side.
1.3.5 Watertube low pressure casing HRSG-A multiple tube circuit
heat exchanger within a gas-containing casing in which steam is
generated inside the tubes by heat transferred from a hot gas
flowing over the tubes.
1.3.6 Watertube pipe coil HRSG in a pressure vessel-A tube or
pipe coil circuit within a pressure vessel in which steam is
generated inside the tubes by heat transferred from a high
temperature fluid or fluidized solids surrounding the tube
circuits.
1
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A P I PUBL853Y 95 m 0732290 05Y2777 095 m
2 API PUBLICATION 534
SECTION 2-FIRETUBE HEAT RECOVERY STEAM GENERATORS
2.1 General A firetube HRSG produces steam from boiler feedwater
in
contact with the outside tube surface, while cooling a hot fluid
which passes through the tubes. The hot fluid is often a high
temperature gas resulting from combustion or other chemical
reaction. Moderate temperature gases, liquids, and slumes are also
used.
High temperature severe service firetube HRSGs are sup- plied
with boiler water in substantial excess of that vapor- ized.
Natural (thermosiphon) or forced (pumped) circulation systems are
employed. Boiler feedwater is introduced to an overhead steam drum,
which provides for water storage and steam-water separation in
addition to the static head driving force for natural circulation
systems.
Less severe service lower temperature firetube HRSGs are often
once-through (nonrecirculating) kettle boilers. Figures 1 and 2
illustrate horizontal and vertical units involv- ing natural
circulation from an overhead drum. Figure 3 is a kettle steam
generator.
2.2 Application 2.2.1 HIGH TEMPERATURUHIGH FLUX UNITS
Firetube HRSGs with high temperature process fluids (ex- ceeding
900F) resulting in high boiling flux rates (in excess of 30,000 Btu
per hour per square foot) are considered severe service
applications. Gas temperatures exceeding 2000F and flux rates to
100,000 Btu per hour per square foot can be ac- commodated in
firetube HRSGs. Mechanical features as de- scribed in 2.6. l are
required for these severe services.
The following process applications are typical of those which
often make use of severe service firetube HRSGs:
a. Steam reformer effluent (hydrogen, methanol, ammonia
plants).
b. Ethylene plant furnace effluent. c. Fluid catalytic cracker
flue gas. d. Sulfur plant reaction furnace effluent. e. Coal
gasifier effluent. f. Sulfuric and nitric acid reaction gases.
Typical steam-side operating pressures range from as low as 150
pounds per square inch for fluid catalytic cracker and sulfur plant
applications to as high as 1800 pounds per square inch for ammonia
and ethylene facilities.
2.2.2 MODERATE TEMPERATURULOW FLUX UNITS
Firetube HRSGs, which handle hot fluid temperatures not
exceeding 900F with flux rates of 30,000 Btu per hour per square
foot and below, have a wide range of process applica- tions. Any
hot fluid stream with a temperature sufficiently above the steam
saturation temperature can be utilized. Qpical process applications
include:
a. Fluid catalytic cracking unit slurries. b. Miscellaneous
refinery hot oil and vapor streams. c. Sulfur recovery
condensers.
Steam-side operating pressures range from 50 pounds per square
inch to 600 pounds per square inch.
2.3 System Consideration 2.3.1 PROCESS FLUID
The thermal-hydraulic performance and mechanical con- struction
of the equipment to a large degree are dependent on specific
characteristics of the hot process fluid. Each process fluid has
unique aspects which must be accounted for in the firetube boiler
design to ensure reliable operation. For example, process fluid
hydrogen content may significantly increase flux.
Riser
Downcomer
Hot fluid out
Hot fluid in -W I I ( I I I I , I I
Exchanger
Figure 1-Horizontal Firetube with External Drum HRSG
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A P I PUBL*534 95 m 0732290 0542778 T21 m
HEAT RECOVERY STEAM GENERATORS 3
2.3.1.1 Fouling
Fouling of the tube inside surface in firetube HRSGs is largely
a function of the specific process fluid. It is also de- pendent on
velocity, residence time, tube size and orienta- tion, and wall
temperature.
Examples of specific concerns include:
a. Ethylene furnace effluent quench coolers are subject to coke
deposition due to continuation of the cracking process at elevated
temperature. Therefore, high gas velocities result- ing in minimum
residence time at temperature are used. b. Hydrogen plant
steam/hydrocarbon reformer effluent heat recovery boilers are
subject to silica fouling when improper refractories are used in
the upstream secondary reformer (for ammonia facilities), transfer
lines, or boiler inlet channels. c. Fluid catalytic cracking slurry
steam generators are generally designed for a velocity of 5 to 7
feet per second to avoid settling out the solid constituents. d.
Fluid catalytic cracking flue gas HRSGs tend to foul with catalyst
deposits.
2.3.1.2 Velocity
The fluid velocity inside the tubes must meet certain min- imum
criteria for the specific processes as noted under 2.3.1.1. There
are also maximum velocity limitations with respect to the erosive
nature of particulate bearing streams. In most cases, however, the
velocity is set by maximum pressure drop or by maximum allowable
heat flux limits which must be considered in design.
2.3.1.3 Pressure Drop
Pressure losses across the tube side of a firetube HRSG are
limited by overall system considerations. For instance,
/ Riser , Hot fluid in
Downcomer
Figure 2-Vertical Firetube with External Drum HRSG
the performance of an olefins plant cracking furnace is pe-
nalized by excessive backpressure imposed by downstream firetube
quench coolers. Sulfur recovery condensers are nor- mally designed
for pressure losses of 1 pound per square inch or less, due to the
low operating pressure level.
2.3.1.4 Temperature Approach
The degree to which the hot process fluid is required to
approach the steam saturation temperature strongly affects the HRSG
size. The approach is defined as the difference between the gas
outlet temperature and the saturated steam temperature. As the
design approach is reduced, the surface area requirement increases.
HRSGs with large approaches tend to use larger diameter or shorter
tubes than those with close approaches.
2.3.1.5 Outlet Temperature Control
Certain process applications require close control of the
process fluid outlet temperature. For instance, secondary re-
former effluent in an ammonia plant enters a CO to CO, shift
reactor after being cooled by the firetube HRSG. Overcool- ing by
the HRSG adversely affects the shift reaction catalyst. For this
reason such firetube HRSGs incorporate a hot gas bypass system,
which may be either internal or external. Re- fer to 2.6. l. 1 1
for further construction details.
The amount of gas bypassed is a function of turndown, ex- tent
of fouling, and the design temperature approach. The equipment
tends to overcool the process fluid when run at reduced throughput
and when clean. HRSGs with large design approaches tend to overcool
due to the large thermal driving force at the outlet end. Such
units require large bypass systems for temperature control to
handle significant bypass fractions.
2.3.1.6 Gas Dew Point
Hot gas streams which may reach the dew point of one of the gas
constituents require special attention. Condensation can occur on
cold surfaces, such as the tubes and refractory lined walls, even
though the bulk gas temperature may be above the dew point. If bulk
gas cooling below the dew point occurs, as in sulfur recovery
boilers, provision must be made to ensure condensate removal.
2.3.2 BOILER FEEDWATEWSTEAM
Appendices A and B provide general information with re- gard to
the boiler feedwater/steam system. Additional con- siderations
unique to firetube equipment are covered in 2.3.2.1 and
2.3.2.2.
2.3.2.1 Heat Flux
Maximum allowable heat flux rates for firetube HRSGs are a
function of equipment construction details, steam
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A P I PUBL*534 95 m 0732290 0542779 968 H
4 API PUBLICATION 534
pressure, recirculation rates, and water quality. Specific con-
by deflector plates or dry pipes. Refer to 2.6.2.4 for addi-
struction features which affect flux limits include: tional shell
details.
a. Tube quantity, diameter and pitch; in general, flux limits
are lower for increasing tube quantity or decreasing pitch to 2.4
Advantages of Firetube Over diameter ratio. Watertube HRSGs b.
Quantity, size, and location of risers and downcomers. 2.4.1 EASE
OF CLEANING c. Clearance between bundle and shell.
Tubes containing fouling prone hot process streams such Actual
flux rates for comparison with design limits are as olefins plant
cracking furnace effluent, coal gasifier over-
based on clean tube surface at the tube inlet where the head,
and fluid catalytic cracking flue gas are easier to clean process
fluid is the hottest. Firetube HRSG design should ac- in firetube
HRSG~. count for increased hot process fluid heat transfer coeffi-
cients due to tube entrance effects. 2.4.2 RESIDENCE TIME
2.3.2.2 Boiler Water Circulation
Critical service high temperature firetube HRSGs are furnished
with elevated steam drums, from which boiler water is supplied with
substantial excess recirculation rates. Systems may be either
natural (most common) or forced circulation.
Low flux HRSGs may also be furnished with an exter- nal drum.
However, such HRSG equipment more com- monly makes use of an
expanded shell side compartment with the tube bundle submerged in
the boiler water vol- ume. Liquid disengagement occurs above the
established liquid level within the expanded shell. Such a unit is
com- monly referred to as a kettle boiler. Natural circulation pat-
terns occur within the kettle shell. A water-steam mixture rises
through the tube bundle; the vapor rises through the steam/water
interface to the steam space above; and the boiler water
recirculates back down each side of the bun- dle to the bottom of
the shell. The kettle HRSG shell serves the purposes of a steam
drum in a conventional boiler system. It differs from a
conventional drum in that the HRSG heating surface is self
contained, connections
Firetube HRSGs have lower process fluid volume and residence
time for services where time at temperature is a factor.
2.4.3 HIGH PRESSURE OR HIGH TEMPERATURE PROCESS FLUIDS OR
SPECIAL METALLURGY REQUIREMENTS
High pressure process fluids contained on the tube side may
minimize HRSG weight. This is particularly beneficial when alloy
materials are used. For example, ammonia con- verter effluent can
reach 5000 pounds per square inch and re- quires alloy or clad
materials. For this case a firetube HRSC may be preferred.
2.4.4 VIBRATION
Firetube HRSGs are less susceptible to damaging flow- induced
tube vibration or acoustic vibration when cooling large volumetric
flow rate gas streams.
2.4.5 REFRACTORY LINING
are altered, and steam/water internal flow patterns are dif-
Elevated temperature gas which requires insulating refrac- ferent.
Saturated steam generated in kettle HRSGs is nor- tory to avoid
overheating pressure bearing components is of- mally used for
process or heating purposes. For such cases ten best handled in
firetube equipment. This is particularly the requirements for
purity and quality (see Appendix A) true for pressurized gas
streams, which cannot be handled in are not high. Therefore,
separation is commonly achieved rectangular duct enclosures.
Refractory lining in firetube
Liquid level , / Steam space
Hot fluid in
Hot fluid out
Figure 3"Kettle HRSG
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A P I PUBL*534 95 m 0732290 0542780 bBT
HEAT RECOVERY STEAM GENERATORS 5
HRSGs is generally required only in the inlet channel com-
partment. In comparison, shell and tube watertube HRSGs require
more extensive refractory linings, which must be en- gineered to
accommodate bundle insertion and removal.
2.4.6 LOW THROUGHPUT ATMOSPHERIC PRESSURE FLUE GASES
Firetube HRSGs are better suited for incinerators and other
combustion systems producing relatively low flow rates of flue
gas.
2.4.7 COMPACT DESIGN
Firetube HRSGs normally require less plot space due to compact
design.
Horizontal firetube HRSGs with an external steam drum can have
the drum mounted on the shell. The drum is sup- ported by the
interconnecting risers and downcomers, thereby eliminating costs
associated with independent support.
2.5 Disadvantages of Firetube Relative to Watertube HRSGs
2.5.1 HIGH THROUGHPUT ATMOSPHERIC PRESSURE FLUE GASES
Firetube HRSGs are not well suited for handling large flow
volumes of near atmospheric pressure gases. Streams such as gas
turbine exhaust require large cross-sectional flow area as provided
by watertube coils installed in rectangular duct enclosures.
2.5.2 LOWER HEAT TRANSFER COEFFlClENTS
Heat transfer coefficients for flow inside tubes are gener- ally
lower than for flow across the tube banks. For this rea- son
firetube HRSGs tend to require more bare tube surface than
watertube HRSGs.
The use of extended surface (fins) against a low pressure
process gas can be an effective means of reducing size. This option
is often utilized in watertube HRSGs, but is generally considered
impractical for firetube designs.
2.5.3 HIGH PRESSURE STEAM APPLICATIONS
For cases involving high pressure steam, typically 1500 pounds
per square inch and above, firetube HRSCs require heavy wall shell
cylinders and tubes. This is particularly true for high capacity
systems. For this reason firetube HRSGs in high pressure steam
systems weigh more than their water- tube counterparts.
2.5.4 HOT TUBESHEET CONSTRUCTION
The hot tubesheet design of firetube HRSGs, particularly its
attachment to the shell and the tubes, may be complex.
The severity of service relates to the coexistence of multiple
conditions, such as:
a. High inlet gas temperature. b. High pressure on the steam
side. c. Loading imposed by the tubes due to axial differential
thermal growth relative to the shell. d. Potential erosive effects
of particulate bearing gases. e. Potential for corrosive attack
from the process and steam sides.
The tubesheet is commonly made of Cr-Mo femtic steels which
require special attention during fabrication and test- ing. Many
firetube HRSGs require a thermal and stress analysis to prove the
construction acceptable for all antici- pated operating
conditions.
2.6 Mechanical Description 2.6.1 HIGH TEMPERATURWHIGH FLUX
UNITS
2.6.1.1 Refractory Lined Inlet Channel
Inlet channels of high temperature units are internally
refractory lined to insulate the pressure components. A num- ber of
refractory systems are available, including dual and monolithic
layers, cast and gunned, or with and without internal liners.
Various types of refractory anchoring systems are also used.
Metallic needles may be considered to further reinforce the
castable.
The selection of refractory materials and their application
method must be compatible with the process service condi- tions.
The design must account for concerns such as:
a. Insulating capability, including effect of hydrogen con- tent
on the refractory thermal conductivity. b. Chemical compatibility
with the process fluid. c. Gas dew point relative to cold face
temperature. d. Erosion resistance against particulate bearing
streams. e. Potential for coking under metallic liners.
2.6.1.2 Channels
Several channel construction options exist. The gas con-
nections may be in-line axial or installed radially on a straight
channel section. Access into the channel compartment is generally
through a manway in large diameter units, or through a full access
cover in small units.
2.6.1.3 Tubesheets
The single most distinguishing feature of high temperature
firetube HRSGs is the thin tubesheet construction. Conven- tional
shell and tube exchangers operating at moderate tem- peratures
incorporate tubesheets designed according to the requirements of
TEMA. Typical tubesheet thicknesses in such units range from 2
inches to 6 inches or more. Use of
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6 API PUBLICATION 534
TEMA tubesheets in high temperature high flux (severe service)
firetube HRSGs is not recommended because the tubesheet metal
temperature gradient would be excessive and high stresses would
result.
The thin tubesheet design is based on the use of the tubes as
stays to provide the necessary support for the tubesheets.
Tubesheet thicknesses typically range from 5 / 8 inch to 1 /*
inches. Flat portions of the tubesheets without tubes must be
supported by supplementary stays.
Sufficient cooling of the tubesheet depends on efficient heat
transfer at the tubesheet backface by shell side vaporiza- tion of
water and high local circulation rate. This offsets the heat input
from the gas through the front face and, more im- portantly, the
area created by all the tube hole perforations.
The steady state tubesheet temperature is dependent on the tube
pitch to diameter ratio and the tubesheet thickness.
Tubesheet temperature can be further minimized by limit- ing
heat flow to the tubesheet with the use of insulated fer- rules
inserted in each tube inlet. The ferrules project 3 inches to 4
inches from the tubesheet face. The space between the ferrules is
packed with refractory, which secures the ferrules and insulates
the tubesheet face. Ferrules are either a high temperature
resistant metallic or ceramic material, wrapped with an insulating
paper for a lightly snug fit in the tube bore. Overcompression of
the insulation will reduce its ef- fectiveness. Figures 4 and 5
show details of one style each of a metallic and ceramic ferrule.
Other configurations have been used.
Insulating material
Metal ferrule Tube
Locating pin
Refractory Tubesheet
Figure 4-Insulated Metal Ferrule
Ceramic ferrule
Refractory /
d
v Tubesheet
Figure 5-Insulated Ceramic Ferrule
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2.6.1.4 Tube-to-Tubesheet Joints
The tube-to-tubesheet joints must provide a positive seal
between the process fluid and the water-steam mixture under all
operating conditions and the resulting pressure and thermal loads.
The joints must also withstand transient and cyclic conditions.
Tube-to-tubesheet joints in severe service applications are
typically strength welded using one of the following
configurations:
a. Front (tubeside) face weld The tubesheet may be J-groove
beveled (see Figure 6A) or the tube may be projected from the flat
face, then welded with a multiple pass fillet (see Fig- ure 6B).
Additionally, each tube is pressure expanded through the thickness
of the tubesheet except near the weld. Such joints may be used in
elevated gas temperature applica- tions generating steam at
pressures to approximately 1000 pounds per square inch. b. Full
depth weld: A deep J-groove with minimum thick- ness backside land
is welded out with multiple passes as shown in Figure 7. If the
land is consumed and fused, the tube and tubesheet become integral
through the full tubesheet thickness. Full depth welded joints are
often spec- ified for high temperature gases generating steam at
pres- sures above 1000 pounds per square inch. c. Back (shell side)
face weld: This type of joint is often called an internal bore
weld. The welding is performed by reaching through the tubesheet
tube hole. It has been applied to a wide range of firetube HRSG
operating conditions, in- cluding high pressure steam systems. A
particular character- istic of this joint is that its integrity is
clearly verifiable by radiographic examination.
A distinct advantage of the full depth and internal bore joints
is their lack of a crevice between the tubesheet and tube outer
surface. A crevice, if present, is subject to accumulation of
boiler water impurities. In high temperature service the
Weld Tubesheet
J-GROOVE BEVELED TUBESHEET
Figure 6"Conventior
insulating effect of a buildup of such material can result in
crevice corrosion and mechanical failure of the joint.
2.6.1.5 Tubesheet Peripheral Knuckle
A thin tubesheet is generally attached to the shell with a pe-
ripheral knuckle between the flat (tubed) portion and the point of
attachment to the outer shell (see Figure 8). The knuckle provides
this critical joint with necessary flexibility to absorb the axial
differential movement between tubes and shell caused by operating
temperatures and pressures. Proper design of the knuckle is
essential for reliable operation of a firetube boiler.
The most severe cases are those involving elevated tem- perature
gases with high heat transfer rates and with high steam side
pressure. Such conditions impose considerable loads on the
knuckles. An example of a severe service application would be
reformer effluent in a hydrogen plant used to produce 1500 pounds
per square inch steam. Exam- ples of less severe services include
fluid catalytic cracking flue gas and sulfur recovery plant tail
gas where condensers generate steam at 600 pounds per square inch
and below.
2.6.1 -6 Channel-Tubesheet-Shell Interconnection
Numerous configurations are available for the intercon- nection
of the thin tubesheet with the HRSG shell and the gas inlet
channel. Figures 8A through 8H illustrate a number of these.
Selection depends on factors such as:
a. Extent of tube versus shell differential thermal growth. b.
Steam pressure. c. Process gas pressure. d. Materials of
construction. e. Vertical versus horizontal HRSG orientation.
Joints shown in Figures 8A and 8B are used for mild services
only, due to the fillet weld attachment and accom- panying crevice.
Figures 8C through 8F all have butt
Tube Tube
Weld Tubesheet
MULTIPLE PASS FILLET
7al Strength Weld
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welded attachment to the shell. The flanged construction of
Figure 8F permits channel removal. Figure 8G is used for high
pressure steam service and Figure 8H is well-suited for vertically
installed units.
2.6.1.7 Tubesheet Without Peripheral Knuckle Configuration
A proprietary firetube HRSG design utilizes a stiff- ened thin
tubesheet which eliminates the peripheral knuckle. Rather than
relieving the tube axial loads with flexible knuckles, the loads
are transmitted directly to the HRSG shell through a stiffening
system which backs up the thin tubesheet. This design may permit
the use of longer tubes. The differential movement absorbed by the
knuckles of a conventional firetube HRSG tubesheet is proportional
to the tube length. For such HRSGs a length limit exists, beyond
which the knuckles would be inca- pable of accepting the imposed
loads within stress limits of the material.
2.6.1.8 Dual Compartment Firetube HRSGs
The length limitation described in 2.6.1.7 is of significant
concern primarily with high temperature, high flux, high steam
pressure equipment. For such Lases the option exists to use dual
compartment construction. Two firetube HRSGs, each with
conventional knuckled tubesheets, are installed in series, as shown
by Figure 9.
The two compartments may be served by a common steam drum.
Advantages of this configuration include:
a. Reduces differential growth between shell and tubes within
each compartment. b. Permits optimization of heat transfer surface
through uti- lization of different tube diameters and lengths in
each com- partment, thereby reducing the total surface
required.
c. Permits locating the internal bypass system in the second
compartment, thereby subjecting the control components to less
severe temperature conditions.
2.6.1.9 Tubes
Typical tube diameters in high temperature firetube HRSGs range
from 1.25 inches to 4 inches. Use of relatively large tubes permits
the following:
a. Low pressure drop application typical of low pressure process
gas streams such as tail gas of sulfur recovery plants. b. Thermal
design at lower heat fluxes. c. Installation of tube inlet ferrules
without over-restricting the flow area available at each tube
entrance. d. Limits the potential for plugging tubes in services
prone to fouling.
The minimum tube wall thickness is governed by applic- able code
rules. Except for cases involving very high process gas pressures,
the steam pressure which acts externally gen- erally controls the
minimum tube thickness.
2.6.1.1 O Tube Arrangement and Spacing
Tubes are normally arranged on a triangular pattern, al-
The selection of tube pitch should address the following though
square layouts may also be used.
concerns:
a. The maximum allowed heat flux is a function of the tube pitch
to diameter ratio. Decreasing the pitch to diameter ratio reduces
the allowable design flux. b. The tubesheet metal temperature is
also dependent on the tube pitch. Decreasing the pitch increases
the metal temperature. c. A minimum tubesheet ligament width
between adjacent tubes is required for welded tube ends to
physically accom- modate the tubesheet J-groove weld preparations.
This is particularly significant for full depth welded joints.
2.6.1 .ll Multiple Tube Passes
Most high temperature process firetube HRSGs are of single tube
pass construction. However, multiple pass tubes may De considered
for processes involving near atmospheric pressure gases used to
generate low pressure steam. The low
Tube heat transfer coefficients characteristically associated
with such gases result in tube metal temperatures which very
Weld closely approach the steam saturation temperature. There-
fore, the metal temperature difference and differential thernlal
growth of tubes of different passes are minimal. Hot pass tubes are
typically larger diameter than subsequent passes in order to
optimize heat transfer within pressure drop constraints. Figure 10
illustrates a two tube pass high tem-
Weld groove
Tubesheet
Figure 7-Full Depth Strength Weld perature firetube steam
generator.
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HEAT RECOVERY STEAM GENERATORS 9
Channel( tYP
Ring (typical)
Hot fluid + Hot fluid +
Tube sheet (typical)
+ E (G) (H)
Figure 8-Channel-Tubesheet-Shell Interconnection
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2.6.1.12 Gas Bypass Systems refractory lined or provided with
internals to preclude the possibility of impingement of hot bypass
gas on the channel
Gas bypass 'ystems for Outlet temperature wall. A typical
internal bypass system is shown in Figure 11. may be external or
internal to the boiler. Internal bypasses Other systems are
available. are commonly used because they take advantage of cooling
the bypass pipe with boiler water. The pipe maybe internally
insulated to ensure that the metal temperature is maintained close
to the water temperature. In high steam pressure appli- cations the
pipe may be attached to a transition knuckle in each tubesheet to
absorb axial loads. The pipe is located in the center of the tube
layout to provide for axisymmetric dis- tribution of loads.
An automatically controlled valve is furnished at the out- let
end of the gas bypass pipe. To reduce the size of the pipe and
valve and to increase the flow control range, a plate with
adjustable dampers may be installed in the outlet channel. By
setting the dampers to a more closed position, the addi- tional
pressure drop imparted to the main gas stream encour- ages flow
through the bypass. The outlet channel should be
2.6.1.13 Risers and Downcomers
Adequate quantity, size, and proper location of risers and
downcomers are essential for reliable operation of high
temperature, high flux firetube HRSGs. Setting the steam drum
elevation, sizing the interconnecting circulation pip- ing, and
positioning the connections are an integral part of the design.
Riser and downcomer design and connection positioning depend on
boiler orientation. Horizontal firetube HRSGs are usually furnished
with multiple risers and downcomers. Connections are positioned to
serve zones of equal steam generating capacity. For single pass
boilers the connections tend to be more closely spaced at the hot
end. At least one
Riser outlets (typical)
Intermediate channel
Downcomer inlets (typical)
Figure %Dual Compartment Firetube HRSG
r Risers Hot fluid out
+------ Downcorner
Hot fluid in "e -? \ Inlet channel Return channel 1
Figure 10-Two Tube Pass Firetube HRSG
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riser and downcomer pair should be located as close as pos-
sible to the hot tubesheet.
Vertical units have one or more downcomer connections located at
the bottom of the boiler shell. Of greater signifi- cance, however,
is the construction at the top which must ensure ample and
continuous wetting of the entire shell side face of the tubesheet.
The following construction options may be considered to help avoid
vapor blanketing beneath the upper tubesheet:
a. Multiple riser connections installed around the full cir-
cumference as high on the shell as possible. b. Reverse knuckle
tubesheets to permit further elevation of the riser connections
relative to the tubesheet. Refer to Figure 8H. c. Special baffling
under the tubesheet to direct water across the back face of the
tubesheet. d. Special formed or machined upper tubesheet with a
slight taper from the center upward to the periphery. e.
Installation of the entire boiler slightly canted from true
vertical so that the tubesheet slopes slightly upward toward the
risers which are located on that side.
2.6.2 KETTLE STEAM GENERATORS
Kettle steam generators are horizontally installed units with an
enlarged shell side boiling compartment diameter relative to the
tube bundle. The bundle penetrates through ei- ther a port opening
in a conventional head, or the small end of an eccentric conical
transition. The latter design is more common.
2.6.2.1 Tube Bundle Construction
Tube bundles may be removable or fixed. Removable bundles offer
certain advantages. The bundle may be re- moved for inspection,
cleaning, repair, or replacement. Also, removable bundles avoid the
differential axial thermal ex- pansion stress which occurs in fixed
tubesheet designs.
Removable bundles may be of either U-tube or floating head
construction. For fluids prone to fouling or erosive process fluids
that may require mechanical cleaning or in- spection, the floating
head type is preferred.
2.6.2.2 Tube Size, Arrangement, and Number of Passes
Typical tube diameters are 3/4 inch and 1 inch, although larger
sizes are considered for highly prone to fouling or vis- cous
process fluids such as in sulfur condensers. Tubes are arranged on
either a square or triangular pattern. The square arrangement is
used if cleaning of the outside tube surface is anticipated, as
could be the case for generating low pressure steam from poor
quality boiler water. In such cases y4 inch minimum width cleaning
lanes are maintained between tubes. Otherwise, a pitch to diameter
ratio of 1.25 is nor- mally used, unless heat flux considerations
require a more extended spacing.
Multiple tube passes may be used for all bundle types described
in 2.6.2.1, except for cases with extremely long hot fluid cooling
ranges which may experience severe ther- mal stress. Single pass
tubes are basically limited to fixed tubesheet construction.
2.6.2.3 Channel Construction
The selection depends primarily on the anticipated frequency of
opening the unit for inspection or cleaning. If frequent access is
required, a channel with bolted cover plate is desirable. Channels
may be any of the TEMA designated types.
2.6.2.4 Shell Construction for De-Entrainment
A degree of disengagement of liquid is achieved in the steam
space above the liquid level. The effectiveness of this volume is a
function of the free height available. A typ- ical minimum height
is 20 inches in steam generating equipment. Units which produce
very low pressure steam or operate at relatively high flux tend to
need additional height. Simple dry pipe devices are sometimes used
to en- hance separation.
A properly sized kettle shell produces steam of adequate quality
or purity for most process and heating applications. Higher punty
steam may be achieved by the installation of separators in the
vapor space above the liquid level, within a
4 Hot fluid out I
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dome welded to the top of the kettle, or in the exit vapor line.
Types of separators include:
a. Wire mesh pads. b. Chevrons. c. Cyclones. d. Combinations of
Items a, b, and c.
Refer to Appendix A for further information.
2.6.3 OTHER TYPES OF FIRETUBE HRSGS
There are many other types of firetube HRSGs designed for a
variety of services. They may be further classified as follows:
a. Proprietary designs developed for specific process appli-
cations. b. Boilers designed with TEMA tubesheets and external
drums. The boilers may be installed in the horizontal or ver- tical
position. c. Partially tubed horizontally installed boilers as
shown in Figure 12. Tubes omitted from the top portion of the
tubesheets provide the steam space for internal disengage- ment.
The channel diameter is larger and the shell diameter is smaller
than those of kettle HRSGs. Tubesheets may be TEMA, or stayed
thin.
2.6.4 CODE CONSIDERATIONS
Firetube HRSGs are designed in accordance with either ASME
Boiler and Pressure Vessel Code, Section I or Sec- tion VIII,
Division I .
Heavy tubesheet firetube HRSGs are normally designed to TEMA
requirements. ABMA guidelines are commonly followed for boiler
feedwater treatment, allowable concen- tration of boiler water
dissolved solids, blowdown, and steam purity.
2.6.5 CONSTRUCTION MATERIALS
Materials selected for use in firetube HRSGs must be compatible
with the process fluid, the boiler water, and
steam with which they will come into contact. The materials must
also exhibit mechanical properties consistent with the design
requirements of the equipment.
2.6.5.1 Corrosion Resistance
Each process fluid from which heat is being recovered has its
own composition and may therefore have its own unique requirements
for construction materials. An impor- tant factor in materials
selection is often resistance to hy- drogen attack, because many
high temperature process gas streams have significant hydrogen
content. The specifica- tion of materials must also account for the
possibility of gas cooling below its dew point, and the corrosive
acids which may be formed. Cold metal surfaces can cause local
condensation, even though the bulk gas may be above the dew
point.
Pressure components wetted by boiler water, including tubes and
tubesheets, are normally fabricated from ferritic materials. Boiler
shells are generally carbon steel. Materials subject to stress
corrosion cracking, such as austenitic stainless steels, are
normally avoided and are prohibited in the evaporator by ASME
Boiler and Pressure Vessel Code, Section I.
The relative growth of the shell and tubes due to temper- ature
changes is of considerable significance to firetube HRSG design.
Materials with similar coefficients of thermal expansion are
beneficial. This is another reason for avoiding the use of
austenitic tubing.
2.7 Operations Description Safe and reliable operation of
firetube HRSGs depends on
the development and use of good operating procedures, spe- cific
to the process and HRSG design.
2.7.1 PROCESS SIDE OPERATION
2.7.1.1 New refractory lining may require a special heat- ing
sequence on start-up to effect proper dryout.
T Steam separator
Hot fluid in -+ * Figure 12"Partially Tubed Firetube HRSG
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2.7.1.2 Firetube HRSGs must not be subjected to hot gas flow
unless the tube bundle is fully covered by boiler feedwater.
2.7.1.3 The rate of temperature change during transients should
be controlled to minimize the potential for thermal shock.
2.7.1.4 All modes of operation should be evaluated during the
design phase, particularly with regard to the ability of the boiler
components to withstand the primary and secondary stresses during
cyclic operation.
2.7.2 STEAM SIDE OPERATING CONCERNS
2.7.2.1 Reliability of Boiler Feedwater Supply
Of primary importance to the successful operation of firetube
HRSGs is the reliable supply of boiler water to the heat transfer
surface. In the event of boiler feed water supply failure, the
control system must shut off the hot stream flow to the HRSG. Refer
to Appendix A for further information.
2.7.2.2 Boiler Feedwater Treatment
Boiler feedwater chemical treatment must protect boiler
components from water side corrosion. Improper treatment, or
upsets, may cause premature failure. Water treatment specialists
are normally consulted.
2.7.2.3 Continuous Blowdown
Blowdown of boiler water must be used in conjunction with boiler
feedwater treatment to ensure that boiler water impurities are
maintained at or below recommended maxi- mum concentration.
Continuous surface blowdown may be accomplished through a
perforated collector pipe located just below the water-steam
interface or a connection at the shell bottom. Continuous blowdown
from kettle HRSGs should be extracted primarily from the end
opposite the feedwater inlet where impurities would be most
concentrated.
2.7.2.4 Intermittent Blowdown
Intermittent blowdown acts to remove settled accumula- tions of
boiler water solids. Connections are located at low points in the
shell, particularly in the most stagnant regions. Blowdown valves
are operated at prescribed intervals, de- pending on the
effectiveness of boiler water treatment.
2.7.2.5 Liquid Level in Kettles
There is no clearly defined water-steam interface inside the
shell. Steam bubbles rise vigorously through the water from the
heat transfer surfaces. A density difference exists between the two
phase mixture in the boiler shell and the liq- uid in an external
gage glass. To ensure submerged tubes, the water level is normally
maintained at 2 inches to 4 inches above the top of the uppermost
tube row.
SECTION 3-VERTICAL SHELVTUBE WATERTUBE HRSGS
3.1 General 3.1.1 A vertical shell/tube watertube HRSG is a
vertical tube-bundle heat exchanger where steam is generated inside
tubes by a shell-side hot fluid.
3.1.2 Floating head, U-tube, or bayonet and scabbard tube
construction are types of vertical shell/tube watertube HRSGs. See
3.5. Figures 13 and 14 are typical floating head and
bayonet/scabbard types of vertical shell/tube watertube HRSGs.
Other configurations may be used.
3.1.3 Hot fluid can enter the shell from either the top or the
bottom. The shell is usually a one pass shell TEMA type E
arrangement. The steam/water side is a one pass system through the
tube bundle or a two pass system through the U-tube and
bayonet/scabbard designs.
3.1.4 The vertical shell/tube watertube HRSG can be ei- ther
natural circulation (thermosiphon) or forced circulation (pumped).
The U-tube arrangement is always forced circula- tion. For either
flow arrangement, the two phase steam/water mixture is separated in
a steam drum. The separated water is recirculated back to the
tubes. Weight percent of steam
generated ranges from about 5 percent to about 20 percent of the
total water circulated. Refer to Appendices A and B for steam drum
and circulation considerations.
3.2 Application 3.2.1 Vertical shell/tube watertube HRSGs are
typically used to generate steam from hydrocarbon gases or liquids,
catalyst-laden flue gases, and viscous process fluids. Vertical
shell/tube watertube HRSGs are generally suitable for either clean
or fouling shell-side fluids. Highly fouling or very dirty fluids,
however, may be better suited for tube-side de- signs since these
types are more easily cleaned. To facilitate cleaning the
shell-side of bundles in fouling or dirty service, use removable
tube bundles with square or rotated square pitch and Y4 inch
minimum cleaning lanes. Shell-side veloc- ities above 1 foot per
second are preferred to minimize foul- ing. To avoid laminar flow
and low heat transfer coefficients, vertical shell/tube watertube
HRSGs are used where the shell-side fluid viscosity exceeds 10
centipoise.
3.2.2 Any pressure steam may be generated, but these HRSGs are
particularly suitable for high steam pressures.
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3.3 System Consideration steam pressure is 1500 pounds per
square inch or greater and the process hot stream is 600 pounds per
square inch or
3.3" For circu1ation7 the drum must be lower. API Recommended
Practice 521 requires that the low
ating tubes. side design pressure to protect against tube
rupture.
3.3.2 High purity water is particularly important for the As an
alternative to designing the shell side for two-thirds
signs. Deposits in bayonet/scabbard designs might insulate
overpressurization by a relief valve. The shell side design
pres-
are also sensitive to deposits because of difficulty in clean-
lower pressure results in lower weight. ing. See Table A- 1 for
boiler water quality and associated 3.3.4 Special features are
required in design or operation steam purity. to protect the HRSG
in event of water failure.
3.3.3 Vertical shell/tube watertube HRSGs may weigh 3.3.5 The
tube side of U-tube bundles are difficult to clean less than fire
tube designs. Examples are designs where via rodding around the
U-bends. The tubes are susceptible to
higher than the top tube Opening Of any Of the gener- pressure
side design be raised to two-thirds the high pressure
long-term operation and reliability of bayonet/scabbard de- the
high Pressure side fie shell can be protected against
the tips of the tubes causing overheating and failure. U-tubes
sure can then be based On the process stream The
Steam and water out
Top tubesheet
Inlet belt
f- Hot fluid in Inlet tubes and erosion sleeves
Flanges for removable bundles
Longitudinal finned tube bundle Hot pressure shell
(refractory lined)
Bundle supports
Hot fluid out
I I
Floating head
t for expansion BFW in
Figure 13-Vertical Watertube Floating Head HRSG
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scale, deposits, and debris because of the small inside tube
diameter and orientation of the tubes. Such debris may col- lect at
the low point in the U-bend. Downcomer strainers may be installed
to minimize tube plugging from large for- eign objects.
Bayonet tube designs have a restrictive annulus between the
tubes. This requires extra care in keeping debris out of the system
which could block flow to a tube. A strainer is generally required
in the water supply piping at the steam generator entrance in these
cases.
3.4 Advantages of Vertical ShelVTube Watertube Over Firetube
HRSGs
3.4.1 Vertical shellhube watertube HRSGs can be designed for
higher maximum heat flux than firetube HRSGs since flow
distribution and steam generation are
more uniform. See Table B-1 for heat flux comparison of
watertube HRSGs versus firetube HRSGs.
3.4.2 If multiple risers are required, they can each be arranged
to have the same proportion of total flow. In high pressure
systems, equal flow in each riser provides more uni- form
distribution to facilitate better performance of the steam
drum.
3.4.3 The tube bundle is free to expand so stresses from
differential expansion between the tubes and shell are not large.
This is particularly true of the bayonet tube design where each
tube can expand independently without inducing tubesheet stresses
from differential expansion between adja- cent tubes.
3.4.4 The vertical shell/tube watertube HRSG requires no
refractory lining of the tube side channel or floating head
BFW inlet + Top tubesheet
Flanges for removable bundles
Bayonet
Scabbard
Cap
-L c
c
c
c
J
Hot fluid in I -
Steam and water out
Hot fluid out
support lugs
Cross flow baffles
Pressure shell
Figure 14Bayonet Exchanger HRSG
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tubesheet face. In the event of a single pass bare tube failure,
3.5.3 U-TUBES no refractory has to be removed to repair the failed
tube.
3.4.5 If U-tube and bayonet tube designs have tubesheets at the
outlet (cold end) of the shell, the tubesheets can be de- signed
with lower alloy metallurgy than when located at the inlet or hot
end of the shell.
3.4.6 In the event of tube failure for bayonet tube units, the
bayonet tube bundle and tubesheet can be removed to ac- cess the
scabbard tube or tubes that failed.
"
A U-tube vertical shell/tube watertube HRSG consists of a
vertical shell with a channel at the top. Inside is a tradi- tional
U-tube bundle. Water enters the channel, flows down the inlet leg
and up the outlet leg of the U-tube. The heating fluid flows up or
down through the bundle shell side. The U- tube bundle is free to
move in response to thermal growth and is not affected by the shell
thermal growth.
3.5.4 BAYONET TUBES
3.4.7 Spare tube bundles will enable quick turnaround The
bayonet vertical shell/tube watertube HRSG employs since the
damaged bundle can be quickly pulled and the a set of concentric
tubes. The smaller diameter tube is called spare bundle installed
to allow the unit to be put back on the bayonet, and the larger
diameter tube is called the scab- stream. bard. The scabbard tube
pattern may be a standard triangular
or square pitch. Each scabbard tube is inserted through two 3.5
Mechanical Description tubesheet holes and attached. The end of the
scabbard tube is 3.5.1 FLOATING HEAD
The bottom tubesheet is a floating head configuration with a
nozzle that passes from the floating head through the shell head to
allow water to be fed from the bottom. The feed wa- ter is heated
and becomes a two-phase mixture as it passes through the heated
zone and enters the top channel. After one pass through the
exchanger, the steam/water enters a steam drum. The hot fluid may
be concurrent or countercur- rent on the shell side. An expansion
joint in the floating head inlet nozzle and the floating head are
provided to compen- sate for thermal expansion between shell and
tube.
3.5.2 FIXED TUBESHEET
capped. The bayonet tube and tubesheet pattern are identical to
the
scabbard pattern. The bayonet tubes are attached to this
tubesheet and are not capped.
The two bundles are assembled so that each bayonet is inserted
into its corresponding scabbard. The tubesheets are separated
axially by a channel spacer piece. Refer to Figure 14.
3.6 Operations Description The boiler feed water enters the
bayonet channel compart-
ment, flows down through the bayonets, up the scabbard an- nulus
to the lower channel compartment, and then out into a steam/water
riser. The double channel bundle is inserted into a vertical
flanged shell. The process heating stream enters
The shell of the heat exchanger may contain an expansion the
shell at the bottom below the ends of the scabbard bundle joint to
allow for differential thermal growth between the and flows up
through the bundle, exiting the shell at the top tubes and the
shell. just beneath the scabbard bundle tubesheets.
SECTION 4-WATERTUBE COILS INSIDE PRESSURE VESSELS
4.1 General 1400F. This catalyst is a large source of heat to
generate steam within the coil(s).
4.1.1 Watertube coils may be located either horizontally or
vertically inside a vessel. The vessel contents are the heat 4.3
System Consideration source. Figure 15 shows a horizontal
arrangement.
4.3.1 Frequently the coils are designed with multiple tube 4.1.2
The coils are supported inside the vessel at several side flow
passes. In these cases, the cross-section flow area, parallel
levels. the heat transfer surface area of each pass, and the
water
flow rate to each pass should be identical. Design inlet
flow
second.
4.3.2 The heat transfer parameters, such as transfer rate, A
typical example is a cat-cracker catalyst regenerator circulation
ratio, and specific flow rate, must be selected to
4-1 -3 The coils may be fabricated of either pipe Or tubing.
velocity in each pass should be in the range of 3 to 7 feet per
4.2 Application
equipped with a fluidized bed of catalyst at 1300F to ensure
nucleate boiling.
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HEAT RECOVERY STEAM GENERATORS 17
4.3.3 The design of the coils is quite critical. Conservative
practice dictates careful choice of coil inside diameter, length,
and heat flux to arrive at a satisfactory and operable system to
avoid film boiling, vapor lock, or unstable flow conditions. During
process upsets, such as cessation of bed fluidization or
circulation, loss of steam generation may occur in one or more of
the individual parallel passes. The system design shall consider
this possibility.
4.3.4 Forced circulation is recommended to ensure a fixed flow
rate to each coil with a minimum circulation ratio of 5: l .
4.3.5 An annular flow regime is desired. If annular flow cannot
be achieved, special inserts may be installed to ensure wetted
walls.
4.3.6 The system shall be designed with the water side pressure
always being greater than the shell side pressure.
4.3.7 Applications may exist where flow measurement or control
may be required on individual water side passes of multipass coils.
Isolation of individual passes may be consid- ered, although the
loss of cooling may damage the coil.
4.4 Mechanical Description The coil shall be designed to meet
the requirements of the
applicable ASME code. Thermal stresses and mechanical loads must
be considered in designing the coils.
4.5 Operations Description 4.5.1 Proper drum water treatment
must be exercised to avoid having water side deposits. Such
deposits would lead to high tube metal temperatures, loss of
thermal efficiency, and ultimate tube failure. Water side deposits
may also lead to possible pump failure. Where several parallel
steam gen- eration coils are required to make up a total steam
rate, a careful choice of control valve location is necessary to
en- sure balanced flow. Since the heat transfer environment in
which the steam generating coils are located is a severe ser- vice,
orifices balance flow at the inlet of each parallel coil. Control
valves must be provided there also to shut down spe- cific coils in
the event of a break or leak from the coil.
4.5.2 Pressure relieving devices may be required on the vessel
to protect the vessel in the event of coil failure.
I
Vessel wall
I
LL-l
COIL SUPPORT
PLAN VIEW COllS
I ELEVATION VIEW
Figure 15-Pipe Coil HRSG Inside a Pressure Vessel
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SECTION 5-WATERTUBE LOW PRESSURE CASING HRSG
5.1 General pressure steam for injection into combustion
turbines for 5.1.1 TYPICAL SYSTEM
The watertube low pressure casing HRSG generates steam inside a
number of tube circuits that are heated by a hot gas stream flowing
through an enclosure of insulated steel casing plate.
The gas normally flows across the tubes in a single pass from
inlet to outlet. In certain cases, baffles or directional vanes may
be used to direct the gas across the tubes creating additional gas
passes.
Steam generating tubes are connected to drums or head- ers. The
tubes may be arranged in one continuous circuit or may be
manifolded at their inlet and outlet ends to form a number of
parallel flow paths.
Steam drums may be either integral to the steam generat- ing
tube circuit or mounted remotely from the tubes.
Additional tube circuits may be used for preheating feed- water
or superheating steam.
5.2 Application The watertube low pressure casing HRSG is used
to re-
cover heat from low pressure exhaust or flue gases. Some common
applications are:
a. Heat recovery from combustion turbine exhaust gas to produce
steam for use in process(es), in enhanced oil recovery, and in
cogeneration. b. Heat recovery from fired heater flue gas to
produce steam. c. Heat recovery from fluid catalytic cracking
regenerator flue gas to produce process steam.
The casing shall be designed with tight joints, preferably seal
welded, to prevent gas leaks to the atmosphere. Some minor leakage
may occur at casing penetrations where ther- mal growth must be
accomodated. The watertube low pres- sure casing HRSG is not used
in services where leaks of the exhaust or flue gas would not be
permissible.
5.3 System Consideration The watertube low pressure casing HRSG
may generate
steam at a single pressure level or multiple pressure levels.
Generating at multiple pressure levels may increase the ef-
fectiveness of recovering the heat from the gas and will pro- vide
steam for various plant needs.
Combustion turbine HRSGs used in cogeneration or com- bined
cycle power plants often generate steam at two or three pressure
levels. Each pressure level may have its own econ- omizer,
evaporator, or superheater section.
Multiple pressure level combustion turbine HRSGs often produce
high pressure steam for steam turbines, intermediate
NO, control or induction into steam turbines, and low pres- sure
steam for deaerators.
When steam requirements are greater than can be obtained with
the heat available from a combustion turbine exhaust gas,
supplementary firing can be used to increase the avail- able
heat.
If steam is required when the combustion turbine gas flow is
curtailed, fresh-air firing (see 5.6.11) with ambient com- bustion
air can be used to generate steam.
A gas bypass system can be provided to isolate the gas source
from the HRSG. This will allow independent operation of the gas
source or HRSG. A combustion turbine can be started with the gas
bypassed to the atmosphere for rapid start- up. Modulating the gas
can also reduce the thermal shock of start-up to the HRSG or
control the HRSG steam output.
The gas may be slightly above or below atmospheric pressure.
Combustion turbine exhaust static pressures are typically in the
range of 8 inches to 16 inches H20 and the HRSG casing design
pressure is typically 20 inches H20. Fired heater flue gas
pressures are normally slightly below atmospheric pressure. Fluid
catalytic cracking regenerator flue gas pressure may be 1 pound per
square inch gauge or greater. The HRSG casing is normally limited
to gas pres- sures of 5 pounds per square inch gauge or less due to
the practical strength of the gas path enclosure casing.
For combustion turbine Hk'SGs, the gas pressure drop should be
optimized considering the reduction in combustion turbine output
with increased back pressure and the enhanced heat recovery with
increased gas pressure drop in the HRSG.
Combustion turbine exhaust gas temperature may range from 750F
to 1 lOOOF. When supplementary fired, the gas temperature entering
the HRSG may be as high as 1800F without water wall
construction.
The gas temperature exiting the HRSG is normally in the range of
250F to 500F. The exit gas temperature depends on feedwater
temperatures, steam requirements, and corro- sion considerations.
The exit gas temperature should be above the water and acid dew
point of the gas to avoid ex- cessive corrosion of the casing.
To prevent corrosion, the temperature of tube and fin sur- faces
exposed to the gas should be maintained above the water and acid
dew points unless special metallurgy is used.
Feedwater preheat sections are sometimes bypassed (run- dry)
when firing oil to prevent corrosion of the section.
Feedwater preheat coils may contain water that has not been
deaerated. Tube metallurgy other than carbon steel may be
required.
For environmental consideration, HRSGs are often required to
contain emission control equipment such as selective catalytic
reduction systems for NO, control.
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HEAT RECOVERY STEAM GENERATORS 19
Emission control systems are located within the HRSG such that
the flue gas temperature range in the system is correct for a
selective catalytic reduction system operating temperature range.
The location and operating temperature range is influenced by gas
turbine/supplementary firing modes of operation.
When fouling is anticipated, cleaning provisions may be
required. The configuration of the heat transfer surface may have
to be modified to enhance cleaning.
Protection from freezing is especially important for HRSGs used
in cyclic service. Provisions for rapid start-up may also be
included such as steam sparging or steam coil heating in drums to
maintain the pressure components in a warm condition.
5.4 Advantages The HRSG can recover energy otherwise exhausted
to the
atmosphere rather than burning fuel in a fired boiler, reduc-
ing plant fuel consumption and emissions.
The low pressure steel casing design permits very large units
that can handle high volumes of hot gases and produce large
quantities of steam.
The HRSG is capable of rapid start-up. The configuration may be
varied to suit plant equipment
and plot requirements. Different process steam users can be
supplied from one piece of equipment.
Emission control equipment can be incorporated in the low
pressure casing HRSG at optimum operating tempera- tures. Other
HRSG types may not have this capability.
5.5 Disadvantages The watertube low pressure casing HRSG is not
suitable
The HRSG casing is limited to designs requiring 5 pounds
HRSGs not enclosed in buildings must be winterized
for small gas volumes.
per square inch gauge or lower.
when installed in cold climate locations.
5.6 Mechanical Description 5.6.1 HORIZONTAL TUBE EVAPORATOR
The flow within a horizontal tube evaporator normally is forced
circulation as described in Appendix B, and Figures 16 and 17. The
steam drum is mounted remotely from the tubes.
It is possible to establish natural circulation through hori-
zontal tubes by elevating the water outlet from the steam drum
sufficiently above the tubes. However, hydraulic resis- tance and
vapor blanketing in the tubes are potential prob- lems. Forced
circulation flow is generally preferred for horizontal tubes.
5.6.2 VERTICAL TUBE EVAPORATOR
The flow within a vertical tube evaporator normally is nat- ural
circulation as described in Appendix B and Figures 18 and 19.
Downcomers can be external to the gas stream con- necting the upper
drum and lower drum. Downcomers can also be located within the gas
stream. Circulation rates must consider heat input to an internal
downcomer.
I I I + I 1 Feed water e Steam generator
I - 1 J - 1 L
- 1 1 I
I Steam superheater
t t t t Gas
Figure 16-Basic Tubular Arrangement
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API PUBLICATION 534
5.6.3 INCLINED TUBE EVAPORATOR
The flow within inclined tube evaporator arrangements is from a
lower drum or header upward through parallel in- clined tubes to a
collector drum, header, or the steam drum. Natural circulation is
utilized, similar to that described for vertical tubes. The slope
of the tubes and the configuration of the drums, headers, tubes,
and exhaust gas path is critical to proper operation of an inclined
tube arrangement.
5.6.4 PREHEATERS, ECONOMIZERS, AND SUPERHEATERS
In addition to the evaporator, the HRSG may include an
economizer section to heat the feedwater and/or a super- heater
section for superheating steam. (See Figures 16, 17, and 19).
Multiple pressure level HRSGs may have econo- mizers or
superheaters for more than one pressure level.
A steam reheater, feedwater preheater, or a deaerated steam
generating coil may be included at appropriate exhaust gas
temperature locations.
The exhaust gas normally passes over the superheater, steam
generator, and economizer (in that order) to optimize the heat
transfer effectiveness of the HRSG. Alternative arrangements may be
used, such as integrating economizers, evaporators, and
superheaters of different pressure levels to optimize heat transfer
rather than locating the sections of each pressure level together.
Superheater sections may be located within steam generator sections
to limit superheater
tube metal temperatures or the variation of superheated steam
temperature with variations in gas flow or temperature.
5.6.5 STRUCTURE AND CASING
The gas path enclosure is normally a rectangular box en- closed
on four sides with steel casing plate and open on both ends for
entry and exit of the gas stream. Figures 19 and 20 are examples of
gas turbine exhaust and fired heater HRSG layouts, respectively.
Transition ducts are normally provided at the inlet and exit of the
HRSG for connection to the gas source and to a duct or stack
exhausting to the atmosphere or gas cleaning system.
An external structural framework integral to the casing plate
enclosure supports the tubes, headers, and steam drums. Vertical
tubes and their drums or headers may be supported either from the
top or bottom of the structure. Horizontal tubes may be supported
by intermediate and end tube supports.
The gas path enclosure casing plate may be an internal cas- ing
or an external casing depending on whether thermal insu- lation is
installed on the outside or inside of the casing plate.
External pressure casing designs are suitable for gas tem-
peratures to 1800F. The carbon steel casing is protected from hot
gas temperatures by the internal insulation. External cas- ing
designs are suitable for rapid start-up, with start-up rates
dependent on the type of internal insulation system used.
Less common, internal casing designs of carbon steel may be
suitable for gas temperature of approximately 750F or
L
- 1 1 I
I 1 I
3 I
Feed water
I! Steam superheater LL W
u) W
c
9
m 1 I
_ I c
Steam generator I
- 1
Gas
Figure 17-Interlaced Tubular Arrangement
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HEAT RECOVERY STEAM GENERATORS 21
Steam
Feed water-
Feed water
Figure 18-Natural Circulation HRSG
Gas out
Headers
Pressure casing
Water in
SUPERHEATER EVAPORATOR ECONOMIZER
Figure 19-Typical Gas Turbine Exhaust Gas HRSG
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lower. Thermal expansion stresses, thermal growth, and ac-
celerated corrosion of the casing plate are considerations for
internal casing designs. Carbon steel casing plate with in- creased
copper content such as ASTM A 242 or A 588 may be used to improve
corrosion resistance. Rapid start-up with internal casing designs
must consider expansion capabilities due to the thermal stresses
developed between the casing and structural supports or
insulation.
5.6.6 INSULATION
Insulation is provided on the casing to minimize heat loss and
provide personnel protection. If the gas temperature is above 750F,
an internal insulation system is normally used to protect the steel
casing plate.
External insulation is normally block or blanket insulation with
metal lagging for weather protection.
Internal insulation may be castable refractory, ceramic fiber,
mineral wool block, or blanket insulation. '
Internal castable refractory is normally used for fired heater
or fluid catalytic cracking regenerator HRSGs. It is suitable for
high gas velocities and resists erosion from tube cleaning devices.
Various types of castable refractories are available for resistance
to gas erosion and chemical attack. Castable refractories are
suitable for HRSGs, particularly when the firing duct temperature
exceeds 1600"E
Internal block or blanket insulation is normally used for
combustion turbine HRSGs. The insulation is nor- mally a layered
construction with the steel casing on the outside and a system of
steel liner plates on the inside to prevent insulation damage from
gas flow velocities or tube
cleaning devices. The internal steel liner is constructed with
panels overlapping in the direction of exhaust flow held in place
by support pins and washers on the inside sur- face of the liner.
The panels accommodate thermal expan- sion because the edges are
free to slide over each other. Liner materials should be selected
for gas temperatures and corrosion characteristics.
5.6.7 TUBES
Horizontal tube steam generators normally consist of mul- tiple
rows of tubes ranging in size from l inch to 4.5 inches outside
diameter. The tubes may be headered into parallel passes. Tubes
within the same pass are connected in series by return bends.
Natural circulation vertical tube steam generators nor- mally
consist of a group of parallel single tube circuits. Each tube is
connected to the upper drum or header and a lower drum or header.
Water enters each tube at the bottom and flows unobstructed
vertically to the top drum or header. Tubes are normally 2 inches
at the outside diameter.
Forced circulation vertical tube steam generators are com- monly
used for fluid catalytic cracking regenerator HRSGs. Multiple tube
passes are used between the inlet and outlet headers. Tubes are
connected with return bends.
The normal spacing between centers of adjacent tubes is two
times the nominal tube diameter. The tubes in adjacent rows in the
gas path may be arranged in an in-line or stag- gered pattern. For
the same gas flow, staggered arrangements improve heat transfer
over in-line arrangements, but gas pressure drop is increased.
External casing
Steam/water out
Water in
Manifold
lnsulation/refractory
C- Header box
Sootblower lane
Sootblower
Intermediate tube support
Gas
Figure 20-Typical Convection Section HRSG
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Gas side extended surface is used to increase the heat transfer
to the tubes. Welded spiral wound solid or serrated fins are
normally used. Studs may be used when extreme fouling is
expected.
The metallurgy of extended surface is selected for resis- tance
to high temperature oxidation. Recommended maxi- mum temperature
for various materials are shown in Table l.
A minimum temperature limit results from corrosion re-
quirements. Keep the cold end metal temperature above the exhaust
gas dewpoint to prevent acid attack of the metal. Metal
temperatures required to avoid condensation and the resulting
corrosion are s